How can I permanently enable an addon in the userpref through Python?
Enabling the addon modifies the user preferences, those need to be saved before Blender shuts down or the changes are lost the next time Blender starts. You can use bpy.ops.wm.save_userpref() to do so:
import bpy, os
p = os.path.abspath('blenderneuron.zip')
bpy.ops.preferences.addon_install(filepath=p, overwrite=True)
bpy.ops.preferences.addon_enable(module='blenderneuron')
bpy.ops.wm.save_userpref()